||Find a good fixed axle yo-yo for looping. Da Bomb (w/ technic axle), Fast Eddies, Spitfires are what I prefer. Having good yo-yo's to loop with will make your experience a little more enjoyable.
||Shorten the length of the string. A shorter string gives you more control. I usually cut about 1" - 1½" off.
HINT: Keep those broken strings! When I do string tricks, my strings break at the finger most of the time. I usually keep them and re-use it on yo-yo's that I loop with.
||A lot of people will tell you to practice only good loops. I agree with them. Once the loops start going out of control, stop and start again. Try to correct them only when you've become more comfortable with looping.
||The yo-yo should be tilted at the 1 o'clock position when thrown and at all times (will be opposite for left hand). Once it starts to tilt over too much to the right, to the left or straighten out, you'll begin to lose control of it.
||Remember that when you loop with your right hand the string will get looser and it tightens when you loop with your left hand. Knowing how to do "UFO" would be useful since it's used to tighten or loosen the string tension. You can also switch it over to your other hand to loop which also changes the string tension.
||It's all in the wrist and finger. Too much arm action will make it harder to loop.
||It gets frustrating but just keep it at. Stay focused and practice a lot. I abandoned string tricks for a while to practice my loops with my right and left hand. You should do the same. Stop doing string tricks and do nothing but loops. You might as well hit two birds with one stone and practice loops with your weak hand too. I usually do loops with my strong hand and once the string gets too loose, I switch it over to my weak hand and do loops 'til the string tightens and switch it back and forth. When you can loop with one hand, and type a letter with the other, you're almost there.